Direct current power control circuit for use in conjunction with regulated input signal

ABSTRACT

A solid state DC power control circuit to efficiently use DC battery power without the normally associated energy waste which includes an input control signal, amplifying and conditioning circuit (133), a triangular waveform generator circuit (131), a comparator circuit (135), an inverting MOSFET (metal oxide semiconductor field effect transistor) circuit (137), and a plurality of power MOSFET devices (42), which together combine to translate a typical variable resistance control input into a voltage level and condition it suitably to be compared with a known triangular waveform thereby creating a pulse width modulated signal whose pulse widths correspond in duration to the relative percent &#34;on&#34; or &#34;off&#34; of the control input signal and which regulates a plurality of parallel MOSFET devices (42) which together share the load of an external DC power drain circuit (143).

This is a continuation-in-part of copending applications Ser. No.07/707,619 filed on May 29, 1991, , now U.S. Pat. No. 5,179,621, whichis itself a continuation-in-part of U.S. application Ser. No. 07/453,671filed on Dec. 20, 1987, now U.S. Pat. No. 5,029,229.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to direct current power controlcircuits. This invention applies anytime a DC battery is used in anapplication requiring less than constant full battery discharge and,more specifically, to circuits for controlling the speed of motorsassociated with DC battery powered motors or vehicles.

2. Description of the Related Art

The most common method of regulating the speed of a DC electric motortypically involves placing a variable resistor or a sequence of discreteresistors in series with the windings of the electric motor. While thismethod does provide sufficient speed control of the motor it has twodistinct disadvantages. First, the power drawn from the battery in suchan arrangement is not efficiently reduced in direct proportion when thespeed of the motor is reduced. This is because a portion of the power isdissipated through the resistors rather than entirely through the motor.The same current drain occurs on the battery whether the motor is run athigh speed or low speed. The only change is in the relative distributionof the load between the resistors and the motor windings. In someapplications the current drain does decrease as the motor decreasesspeed however the power efficiency of the drain also decreases. A seconddisadvantage which is a by product of the first, is that the powerdissipated through the resistors is given off as heat, which besidesbeing a waste of energy, can be a problem for some applications.

One attempt at a solution to the problem is to utilize solid stateswitching devices to control and regulate the current flow to the motorwindings. One type of solid state switching device capable of handlingthe currents typically required by large electric motors is the SCR(silicon control rectifier). SCRs function much as a mechanical switchmight by allowing a large current to flow between two points when arelatively small voltage is present to toggle the rectifier. In a solidstate circuit a SCR can be pulsed so as to "chop" the current through anelectric motor thereby regulating the current flow and, thus, the motorspeed. While SCRs do have lower energy losses than the resistor controlarrangements they still result in a substantial dissipation of power inthe form of heat. SCRs are also relatively large solid state componentsand are often high in cost. SCRs additionally suffer the drawback ofcharacteristically poor load sharing when placed in parallel across alarge load.

A more promising solid state device that to all appearances functions inmuch the same way as an SCR is the metal oxide semiconductor fieldeffect transistor or MOSFET. Like the SCR a MOSFET allows a relativelylarge current to flow in a circuit when a relatively small "gate"current toggles it "on". Unlike the SCR a MOSFET enjoys the advantagesof having very high input impedance, good load sharing propensity, smallsize, low cost, and very fast switching times.

The fast switching times of MOSFET devices allow them to function atvery high frequencies compared to the slower SCRs. This creates theadded advantage of operation at a level that is above the range of humanhearing and therefore allows relatively quiet control of the electricmotor. The higher frequencies do however have the disadvantage ofcreating larger voltage spikes when the MOSFETs are switched on and off.This would counsel for the use of MOSFETs with higher voltage ratings tohandle these spikes, but with the higher voltage ratings come higherinternal resistances and lower current capacities. One solution to thisproblem is to provide external freewheeling diodes to act as transientsuppression devices across the load.

Another attempted solution is described in Post, U.S. Pat. No.4,626,750. Post's attempted solution is to use an electronic circuitchopper control system to control groups of parallel field effecttransistors and parallel power diodes. The power diodes are distributedapart from each other and among each group of field effect transistors.Post attempts to reduce voltage spikes caused by the field effecttransistors being turned on and off by using the power diodes to drawcounteracting current from the battery. Unfortunately, this arrangementresults in excess heat which must be dissipated through fins, which inturn results in less efficient power drain from the battery. Post alsorequires a complex control circuit to ensure that the system operatessafely within acceptable parameters so as to not damage the electroniccomponents or host vehicle.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a reliable andefficient means of regulating the discharge of a DC battery.

A further object of this invention to provide a reliable and efficientmeans of regulating the speed of DC electric motors of the type commonlyused in electrically powered vehicles.

Another object of this invention to provide a means of regulating thespeed of an electric motor by drawing from the battery only such currentas is required by the motor and without suffering substantial losses ofbattery power through heat dissipation.

It is a further object of this invention to provide a circuit thatemploys solid state MOSFET devices to achieve the above stated objectsand at the same time is capable of functioning at frequencies at orabove human hearing, is capable of handling large currents, is capableof operating across a large range of direct current voltages, and iscapable of limiting transient voltage spikes that occur during switchingso as not to damage the MOSFET devices.

It is also an object of this invention to achieve the above statedobjects through a minimum of circuitry and in a manner that is bothdurable and versatile in its ease of application so as to be used in awide range of applications.

This invention provides a solid state direct current power controlcircuit which includes an input control signal circuit, a waveformgenerator circuit, a comparator circuit, an inverting MOSFET (metaloxide semiconductor field effect transistor) driver circuit, a pluralityof power MOSFET devices, and a transient voltage suppression diode,which together combine to translate a typical variable voltage controlinput signal and compare it with a known waveform thereby creating apulse width modulated signal whose pulse widths correspond in durationto the relative percent "on" or "off" of the control input signal. Thecircuit incorporates a plurality of parallel MOSFET devices whose gatesare regulated by the modulated signal and which together share the loadof an external DC motor circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of the preferred embodiment of theinvention which indicates both functional parts of the overall circuitand some specific components thereof.

FIG. 2 is a graphic representation of the output signal from theoperational amplifier (133) of the circuit in FIG. 1.

FIG. 3 is a graphic representation of the output signal from thetriangular waveform generator (131) of the circuit in FIG. 1

FIG. 4 is a graphic representation of the combination of FIG. 2 and FIG.3.

FIG. 5 is a graphic representation of the output signal from thecomparator (135) of the circuit in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to FIG. 1 for a detailed description of oneembodiment of the design of the present invention. Other embodiments arepossible as the use for controlled, efficient discharge of a DC batterychanges or as the output power requirements change. The circuitdisclosed in FIG. 1 comprises a number of identifiable functional partswhich will first be described generally and interrelated with oneanother and then subsequently be described individually in more specificdetail. FIG. 1 also makes reference to a number of external circuitswhich are not explicitly described. These circuits are not consideredpart of the disclosed embodiment of the invention, but will be describedin sufficient detail to clarify the function of the invention and itsinterrelationship with these circuits.

The circuit in FIG. 1 is composed generally of; a non-invertingoperational amplifier circuit (133), a triangular waveform generatorcircuit (131), a comparator circuit (135), an inverting MOSFET (metaloxide semiconductor field effect transistor) driver circuit (137), aMOSFET network (42), a freewheeling diode (48), and a voltage regulatorcircuit (139). In addition there are two external circuits generallypositioned in FIG. 1 where they would be functionally connected to thecircuit of this invention. These are designated as input control circuit(141) and DC motor circuit (143).

The main function of the disclosed circuit is the regulation of theMOSFET network (42) which in turn regulates the current flow through theexternal motor circuit (143). The current flow through the motor circuit(143) determines the motor speed and thus the speed of the vehicle. Theregulation of the MOSFET network (42) is accomplished by creating amodulated signal whose pulse width reflects the extent to which theexternal input control circuit (141) indicates an "on" or "off"condition. Thus, the ultimate goal of the circuit is to take themechanical displacement of an external motor speed governor (not shown)and translate it into a pulse regulated flow of current through the DCmotor circuit (143).

This control of the DC motor circuit (143) begins in an external inputcontrol circuit (141). The external input control circuit (141) iscapable of supplying input point (8) with a filtered and regulatedvoltage that is indicative of the mechanical position of an externalmotor speed governor (not shown) and therefore of the drive currentdesired on the motor circuit (143). Typically the external input controlcircuit (141) will establish a variable voltage at the input point (8)that decreases when the external speed governor is increasinglydisplaced. This may be accomplished by means of power governancecircuitry of the type disclosed in U.S. patent application Ser. No.07/660,437, now U.S. Pat. No. 5,087,865. The input control circuit (141)should provide a voltage that is indicative of the desired drive on thevehicle/motor.

The non-inverting operational amplifier circuit (133) takes the inputsignal provided by the external input control circuit (141) that isinversely indicative of the desired drive on the vehicle/motor andcompares the voltage to the level and condition of a known waveform. Anexample of the voltage output at point (22) of the operational amplifiercircuit (133) can be graphically seen in FIG. 2. Line 1 in FIG. 2represents the voltage level present when the external motor speedgovernor is at approximately a 20% "on" condition. A 50% "on" conditionwould result in a lower voltage output at point (22) from the amplifiercircuit (133).

The reference waveform required by the comparator circuit (135) can be asymmetrical or asymmetrical waveform. In this embodiment, the referencewaveform is preferably a triangular wave generated in a frequency rangeof at or above 20,000 Hertz so as to be above the level of normal humanhearing. The wave could also be generated below 20,000 Hertz, but thiscould then be heard by humans. Such a waveform is generated by thetriangular waveform generator (131) of FIG. 1. An example of the outputat point (37) of the triangular waveform generator circuit (131) may beseen graphically in FIG. 3. The waveform created is conditioned to becomparable in amplitude to the range of voltage levels provided by theoperational amplifier circuit (133) at point (22) as illustrated in FIG.4.

The output of the operational amplifier circuit (133) is compared to theoutput of the triangular waveform generator circuit (131) by way of thecomparator circuit (135). The comparator circuit (135) has an outputthat flip flops between rails of 12 pk volts DC and 0 volts DC dependingon the relative amplitudes of the two input signals. The amount of timeeach cycle that the output of the comparator circuit (135) holds at 12volts DC is equal to the amount of time each cycle that the triangularwave signal exceeds the amplitude of the operational amplifier outputvoltage. The resulting output of the comparator circuit (135) at point(28) would therefore have a pulsed waveform similar to that shown inFIG. 5. While the period and the frequency of the signal are the same asand are determined by the triangular waveform, the pulse width of thesignal will vary according to the voltage level of the output of theoperational amplifier circuit (133). A high operational amplifiercircuit voltage will result in a small portion of the triangular wavethat exceeds that voltage, and therefore will result in a short 12 voltDC pulse width. It is the pulse width therefore, that is now inverselyrelated to the desired vehicle/motor speed.

To create a direct relationship instead of an inverse relationship andto condition the signal for the MOSFET network (42) an inverting MOSFETdriver (137) is inserted in series at the output of the comparatorcircuit (135). This driver (137) creates a current in a range of 1 ampto 72 amps but in this embodiment it creates up to 6 amps at either the12 volt DC rail or the 0 volt DC rail for input into the MOSFET network(42). The signal is also inverted so that a broader 12 volt pulse widthcorresponds to a greater vehicle/motor speed.

The output signal from the inverting MOSFET driver circuit (137) is thenfed into a network of parallel power MOSFET devices (42). These devices(42) act as switches that open and close in response to the gate currentprovided by the MOSFET driver circuit (137). The pulse width of thesignal from the MOSFET driver (38) therefore determines the pulsedcurrent flow through the drain (49) source (45) connections on theparallel MOSFET devices (42).

This pulsed current flow is directed through the external DC motorcircuit (143) by way of connections (50) and (51), and thereby regulatesthe motor speed. A freewheeling diode (48) placed across the load servesto suppress transient voltage spikes that occur when the MOSFETs arerapidly switched on and off. The freewheeling diode (48) may be placedclose to the motor field windings or may be built as part of the DCpower control circuit. In addition, the diode is preferably a large,hi-power, fast reversing diode.

One ancillary circuit not mentioned above but indicated in FIG. 1 is avoltage regulator circuit (139) that provides the operational 12 voltsDC that the circuit requires from the typical 36 volt DC battery (58)that most electric vehicles use to function. This invention may be usedon batteries ranging from 6 volts to 600 volts with operational volts inthe range of 3 volts to 120 volts.

The invention has been described in relation to use in battery poweredgolf carts, fork lifts' drive battery system, fork lifts' hydraulic liftsystem, and battery powered baggage trucks. However, the invention isusable in a wide variety of applications including, but not limited toboats, trolling motors, arch welding, electroplating, heating,conveyors, and unmanned vehicles.

While the above is a general overview of the circuit as a whole, whatfollows is a more detailed description of the elements of each of thefunctional blocks within the circuit. The specific voltage, current, andresistance numbers should not be construed as maximum or minimum valuesbut only as those values which are used in this preferred embodimentbecause other values could be used with simple electronic circuitcalculations. In addition, the control circuit could be easily made intoan integrated circuit. For example, a pulse width modulated controllerintegrated circuit could include the amplifier, the modulator, theoscillator, and the comparator described above. And, a pulse widthmodulator driver/controller integrated circuit could include theamplifier, the modulator, the oscillator, the comparator, and the driverdescribed above.

The input govenor control circuit (141) creates a voltage at point (8)in a range from 5 to 0 volts DC that varies as the parametersestablished by the input control circuit (141) vary. As mentioned above,the circuitry disclosed in U.S. patent application Ser. No. 07/660,437,now U.S. Pat. No. 5,087,865, is suitable for providing such a voltageinput governor signal in a filtered and regulated form to govern thecircuitry of the present invention. Resistor (14) is a low resistanceinrush current limiter that provides surge current protection for (21)for operation amplifier a controlled change in voltage from 5 volts DCdown to 0 volts DC when the input control circuit (141) voltagedecreases upon "acceleration". Capacitor (5) ties the positive input(20) of operational amplifier (21) through resistor (14) to a steadystate +12 VDC. When power is turned on, capacitor (5) is pulled downfrom this steady state to 5 VDC (input control is at a full offcondition). This prevents operational amplifier (21) from seeing OVDC (afull on condition) momentarily when the power is turned on.

The voltage at point (22) is conditioned by the operational amplifier(21). Feedback resistor (17) provides stabilization for the operationalamplifier (21), and gain resistor (15) regulates the gain of theoperational amplifier (21) by conncting the inverting input (25) toground. Capacitor (19) further filters the voltage signal at the outputof the operational amplifier (21). The output signal at point (22) fromthe non-inverting operational amplifier (21) is represented in FIG. 2.This output voltage can vary from 0 to 12 volts DC depending upon theresistance in the external input control circuit (141).

Parallel to the operational amplifier circuit (133) and above it in FIG.1, is a triangular waveform generator (131). The generator is composedof two operational amplifiers; (111) and (113). The first amplifier(111) is in a non-inverting condition, and the second (113) is in aninverting condition. This combination of operational amplifiers providesan oscillator which generates a triangular waveform at point (37)similar to the graphic depiction in FIG. 3. The capacitors (115) and(117) provide the time constant for the oscillation. The voltage dividernetwork of resistors (123) and (125) provide the biasing of the waveformfor symmetry. Resistor (121) determines the amplitude of the waveform.The output signal at point (37) is pulled up by resistor (35) and theoutput of the non-inverting operational amplifier circuit (133) at point(22) is pulled up by resistor (23). Each of these outputs then becomethe inputs for the comparator circuit (135).

The positive input of the comparator (26) receives the signal from thenon-inverting operational amplifier circuit (133) at point (22) and thenegative input of the comparator (26) receives the signal from thetriangular waveform generator (131) at point (37). The comparator (26)takes the two signals, compares them, and outputs a signal thatflip-flops between a positive nominal voltage of 12 volts DC and anegative nominal voltage of 0 volts DC, depending upon whether thetriangular waveform falls above or below the operational amplifieroutput level at any particular point in the waveform cycle. Thecomparison of these two waveforms can best be seen by superimposing thesignal in FIG. 2, with the signal in FIG. 3. The resulting output of thecomparator (26) can be seen in FIG. 5. The output of the comparator (26)is a sequence of pulses, the widths of which are determined by theamount of time the triangular waveform falls above the operationalamplifier output (duration of the 12 volt pulse) and the amount of timeit falls below the operational amplifier output (duration of the 0 voltpulse). The output (28) of the comparator (26) is pulled up by resistor(31). Feedback resistor (30) is a hysterisis resistor which serves tostabilize the comparator (26).

Up to this point in the circuit the pulsed signal is inversely relatedto the current flow required by the motor circuit (143) foracceleration. In other words, where ever the signal was high, i.e. 12volts DC at the output of the comparator (26), the motor circuit wouldcorrespondingly decelerate or stop. When the signal was low, i.e. 0volts DC, the motor circuit would accelerate or turn on full. At theoutput (28) of the comparator circuit (135) the pulse width modulatedsignal enters an inverting MOSFET driver (38). This MOSFET driver (38)conditions the signal suitably for a plurality of MOSFET devices in theMOSFET network (42). The output at point (40) of the inverting MOSFETdriver provides up to a 6 amp current at 12 volts DC or 0 volts DC forthe upper and lower rails in the pulse width modulated signal.

This output is then paralleled to fifteen MOSFET circuits or one largeMOSFET represented by a schematic of one such circuit generallydescribed as (42) in the diagram. Each of these circuits (42) iscomprised of a resistor (43) and a MOSFET device (44). The MOSFET device(44) has a gate (41), a drain (49), and a source (45). Connected to thisMOSFET circuit (42) is a resistor/capacitor pair (47/46) that serves asa snubber across the drain (49) and source (45) connections on theMOSFET device (44). The outputs of all fourteen of these MOSFET circuits(42) are paralleled together into one end of the external motor circuit(143) at point (51). Between this point (51) and the battery supply (58)at point (50) is included a freewheeling diode (48) for transientvoltage suppression. The MOSFET network (42) is connected to a commonground (56) which is in turn connected to the battery ground (54).

Ancillary to the MOSFET network and the previously described amplifiercircuits is a voltage regulator circuit (139) which includes voltageregulator (60) and capacitors (59) and (61). Capacitor (59) serves tofilter the 36 volts DC off of the battery (58) into the voltageregulator (60). Capacitor (61) serves to filter the 12 volts DC out ofthe voltage regulator (60) to the balance of the circuit, connectionsbeing represented by the Letter "A". The 12 volts DC is provided toevery point so indicated in the circuit.

The MOSFET devices (44) have the distinct advantage of a high inputimpedance, a very fast switching time, a positive temperaturecoefficient of resistance, and a fair degree of automatic load sharing.The type of MOSFET device (44) capable of handling the output of thedriver (38) (6 amps at 12 volts DC) may be found in an IRFZ40 typeMOSFET. This is an "n channel enhancement" MOSFET device. These parallelMOSFET devices thus provide a method for controlling the current flowthrough the motor circuit (143) by way of connections (50) and (51).

The invention is designed to be easily assembled and able to withstandthe rigors of normal use outside of the test environment. Thisdurability is due in part to the simple DC power control circuit and theassembly process. Assembly uses plate technology instead of thetraditional rail technology.

The assembly process (not shown in the drawings) starts with the powerbus being demountably attached to the ground bus. The power bus assemblyis then demountably attached to a plate. Screws are then used to mounteach of the MOSFET's tabs to the plate on both sides of the ground busassembly. The two diodes are then attached to the plate with a screwthrough each diode's end connector for the power bus to be attachedlater and another screw with a lock washer through the center holes forlater mounting of a control board.

The power bus assembly will then be attached across the embodimentbetween the two diodes using the two end screws of the diode. The powerbus assembly is ideally located near the end of the plate. The controlboard then has the two wires that come from the power board attached tothe appropriate points on the control board. Then control board is thenattached to the power bus assembly using the screws that are in thecenter of the diode embodiments with a stand-off brass bushing to holdthe control point at the proper level. The voltage regulator devicewhich is below the control board, will be attached to the ground busassembly next by using the screw through the access hole through theboard. The control board is then tightened down on the top of thediodes. The capacitor will be attached to the diode for the positivelead and the ground bus assembly for the negative lead and will sit atthe end of the device. The enclosure cover will then be lowered over thedevice after applying the proper amount of sealant to the enclosureholes and also a bead around the embodiment. A nut will then be pulleddown on each of the three screws that are protruding from the top of thecover and a nut on the control connector will also be tightened. The DCpower control device is then ready for testing, packaging and shipping.When assembled the device measures approximately 6" wide by 81/2" longand 21/2" high.

Heat dissipation is not lessened for a couple of reasons. First, theamount of heat generated is small because only the amount of energyneeded is being drawn from the battery. Normally, a full load is drawnfrom the battery with the excess energy being dissipated throughresistors in the form of heat. Second, the base plate is actuallyfeeding all the heat. The base plate is being attached to the device andis actually its own heat sink and it sinks into the device's space. Whenattached to the frame of the device, the frame is actually dissipatingthe heat. Therefore, the normally required heat dissipating fins are notrequired.

Although the invention has been described with reference to a specificembodiment, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiment, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of thepresent invention. It is therefore contemplated that the appended claimswill cover such modifications that fall within the true scope of theinvention.

I claim:
 1. A direct current electric motor control circuit for theefficient discharge of a direct current power source through a directcurrent power drain in a manner that allows for longer power source lifeand longer time between power source recharging since power is drawnonly as needed without large amounts of wasted power dissipation throughresistors as heat, comprising:an input connection from an external inputcontrol variable voltage signal circuit; a solid state operationalamplifier circuit having a positive input, a negative input, and anoutput, said operational amplifier circuit in series connection withsaid input connection, said positive input of said operational amplifierbeing in series connection with said input connection, said operationalamplifier having a gain resistor in series connection to a ground, saidoperational amplifier further having a stabilizing feedback resistorconnecting said output to said negative input of said operationalamplifier, said operational amplifier further having an output filteringcapacitor in series connection to a ground; a triangular waveformgenerator circuit comprising a first and a second operational amplifierdevice, said first and second operational amplifier devices each havinga positive input, a negative input, and an output, said firstoperational amplifier device being configured in a non-invertingcondition and said second operational amplifier device being in aninverting condition, said output of said first operational amplifierdevice being in connection with said negative input of said secondoperational amplifier device, said negative input of said firstoperational amplifier device being in connection with said positiveinput of said second operational amplifier device and being further heldat a biasing voltage by a first and a second biasing resistor, saidsecond operational amplifier device further having a first capacitorconnected across said positive and said negative inputs of said secondoperational amplifier device and a second capacitor connected from saidoutput of said second amplifier device to a common ground; a comparatorcircuit comprising a solid state comparator, said comparator devicehaving a positive input, a negative input, and an output, said negativeinput connected to said triangular waveform generator circuit, saidpositive input connected to solid state operational amplifier circuit,said comparator device further having a pullup resistor and a feedbackresistor at said output of said comparator device; an inverting MOSFETdriver circuit comprising a solid state MOSFET driver device, saiddriver device having an input and an output, said input of said driverdevice connected to said comparator circuit; at least one MOSFET circuitcomprising a power MOSFET device and a resistor, said resistor connectedto said driver circuit, said MOSFET device having a gate connection,said gate connection connected to said resistor, said MOSFET devicehaving a source connection, said source connection connected to anexternal power drain circuit, said MOSFET device having a drainconnection, said drain connection connected to a current source ground;a resistor/capacitor pair connected in series, said resistor/capacitorpair connected across said MOSFET circuit; and a freewheeling diodeconnected across said external power drain circuit.